On Windows at least, Opera just passes the mailto URL to whatever program is associated with the mailto protocol. If there's an issue with Opera not passing the mailto URL to your email client, you just go into the Windows settings and make sure the email client is set as the default for the system and that it's specifically set to handle the mailto protocol.

I assume it's the same on Mac. So, I would set your mail client as the default email program again. I guess you have to open the Mac Mail app, goto preferences -> general and set the default email reader. You might then have to restart Opera.

Also, in Opera, goto "Menu -> Settings -> Privacy and Security -> Handlers" and make sure there are no handlers listed there so that there's no site overriding the default behavior.